1. Field of the Invention
The present invention relates to a technology for mounting semiconductor elements to a circuit substrate at a high density.
2. Description of Background Arts
The packaging density of semiconductor elements has been remarkably increased. In order to meet a decrease in packaging area and an increase in number of electrodes, various high-density packaging methods have been proposed in recent years. As one example, there is a method for facedown-mounting semiconductor elements with bumps formed at the electrode section to circuit substrates (U.S. Patent Publication No. 4,661,192, Laid-open Japanese patent publication No. 6-224259). FIG. 1 is a cross-sectional view of a conventional semiconductor device facedown-mounted.
The conventional semiconductor device shown in FIG. 1 comprises a semiconductor element 901, circuit substrate 907, protruding electrode 905 and conductive paste 909 which electrically connects the semiconductor element 901 to circuit substrate 907, and sealing resin 911.
Circuit substrate 907 is, for example, a multilayer circuit substrate with all the layers having an interstitial via hole (IVH) construction, and an electrode 913 is installed for securing electrical connections with semiconductor element 901. To semiconductor element 901, a plurality of electrodes 903 are formed. To each of electrodes 903, protruding electrode 905 is installed, and conductive paste 909 covers part of it. Semiconductor 901 and circuit substrate 907 are electrically connected by pressing conductive paste 909 to electrode 913 by protruding electrode 905. Sealing resin 911 is filled between semiconductor element 901 and circuit substrate 907 as if they fill the clearance of both. Thus, semiconductor element 901 can be fixed to circuit substrate 907.
Referring now FIG. 2A through 2E, a packaging method of the conventional semiconductor device will be specifically described.
FIG. 2A is a diagram showing the semiconductor element. Semiconductor element 901 has electrode 903. First of all, on electrode 903, bump 905 is formed using a wire bonding method. Bump 905 has a 2-level protruding form. The procedure for forming the bump is described as follows. First of all, a ball formed at an Au wire head end is thermally crimped to electrode 903 and the lower level section of a 2-level protrusion is formed. Then, using an Au wire loop formed by moving a capillary, the upper level section is formed. Under this condition, the heights of the 2-level protruded bumps are not uniform and the evenness at the head end section also lacks. Consequently, pressurizing the 2-level protruded bump, the height is uniformalized and the head end section is leveled. In this way, bump 905 is formed on electrode 903.
FIG. 2B is a diagram showing semiconductor element 901 with conductive paste 909 applied. The conductive paste 909 is transferred and formed on bump 905. Specifically, for example, conductive paste 909 is applied on a rotating disk in a uniform thickness using a doctor blade method and against the conductive paste 909 applied, bump 905 is pressed and pulled up to carry out transferring.
FIG. 2C is a diagram showing semiconductor element 901 and circuit substrate 907 before aligning. Aligning is carried out by accurately connecting bump 905 on semiconductor element 901 to electrode 913 on circuit substrate 907.
FIG. 2D is a diagram showing semiconductor element 901 and circuit substrate 907 after aligning. Conductive paste 909 on bump 905 is pressed against electrode 913 on circuit substrate 907 and conductive paste 909 is heated to harden. Thus, bump 905 and electrode 913 are electrically and physically joined.
FIG. 2E is a diagram showing semiconductor element 901 and circuit substrate 907 after sealing with resin 911. Resin 911 is epoxy-based material. To the periphery of semiconductor element 901 and the clearance between semiconductor element 901 and circuit substrate 907, resin 911 is injected and sealing is achieved by hardening resin 911. In this way, by resin-molding circuit substrate 907 and semiconductor element 901, a conventional semiconductor device with semiconductor element 901 flip-chip bonded to circuit substrate 907 is completed.
With respect to FIG. 2B, the amount of conductive paste 909 transferred and formed on each bump 905 is inevitably varied to a certain degree for each bump 905. Consequently, when electrode 903 of semiconductor element 901 and electrode 913 of circuit substrate 907 are electrically connected, pressing bump 905 with a large transferring volume of conductive paste 909 against electrode 913 may cause conductive paste 905 to spread to adjacent electrodes or conductive paste and may result in shorting. This causes problems particularly when the clearances between electrodes 903 and electrodes 913 are narrow.